ENH: reduced verbosity when decomposing/reconstructing empty meshes

- only warn about missing cells/points if the mesh is also missing
  boundary patches.
- reduce verbosity when decomposing to an empty mesh
- skip face matching when either mesh has no faces

applications/utilities/parallelProcessing/decomposePar/domainDecomposition.C

@@ -831,11 +831,18 @@ bool Foam::domainDecomposition::writeDecomposition(const bool decomposeSets)
 
 
         // Statistics
+        Info<< nl << "Processor " << proci;
 
-        Info<< endl
+        if (procMesh.nCells())
-            << "Processor " << proci << nl
+        {
-            << "    Number of cells = " << procMesh.nCells()
+            Info<< nl << "    ";
-            << endl;
+        }
+        else
+        {
+            Info<< ": ";
+        }
+
+        Info<< "Number of cells = " << procMesh.nCells() << nl;
 
         maxProcCells = max(maxProcCells, procMesh.nCells());
 
@@ -865,9 +872,12 @@ bool Foam::domainDecomposition::writeDecomposition(const bool decomposeSets)
             }
         }
 
+        if (procMesh.nCells() && (nBoundaryFaces || nProcFaces))
+        {
             Info<< "    Number of processor patches = " << nProcPatches << nl
                 << "    Number of processor faces = " << nProcFaces << nl
-            << "    Number of boundary faces = " << nBoundaryFaces << endl;
+                << "    Number of boundary faces = " << nBoundaryFaces << nl;
+        }
 
         totProcFaces += nProcFaces;
         totProcPatches += nProcPatches;

src/OpenFOAM/meshes/polyMesh/polyMesh.C

@@ -325,6 +325,11 @@ Foam::polyMesh::polyMesh(const IOobject& io)
     boundary_.calcGeometry();
 
     // Warn if global empty mesh
+    if (returnReduce(boundary_.empty(), orOp<bool>()))
+    {
+        WarningInFunction
+            << "mesh missing boundary on one or more domains" << endl;
+
         if (returnReduce(nPoints(), sumOp<label>()) == 0)
         {
             WarningInFunction
@@ -335,6 +340,7 @@ Foam::polyMesh::polyMesh(const IOobject& io)
             WarningInFunction
                 << "no cells in mesh" << endl;
         }
+    }
 
     // Initialise demand-driven data
     calcDirections();

src/dynamicMesh/polyMeshAdder/faceCoupleInfo.C

@@ -886,6 +886,15 @@ void Foam::faceCoupleInfo::findPerfectMatchingFaces
     labelList& mesh1Faces
 )
 {
+    // Quick check: skip face matching if either mesh has no faces
+    if (!mesh0.nFaces() || !mesh1.nFaces())
+    {
+        mesh0Faces.clear();
+        mesh1Faces.clear();
+
+        return;
+    }
+
     // Face centres of external faces (without invoking
     // mesh.faceCentres since mesh might have been clearedOut)